1. Field of the Invention
The present invention relates to a fuel-injection system having injectors that may inject fuel in accordance with fuel-injection characteristics, which is dependent on operating conditions of an engine.
2. Description of the Prior Art
A fuel-injection system has been well known in which an injector is provided with a needle valve movable in an injector body in a reciprocating manner to open and close injection holes, and a solenoid-operated valve having an electromagnetic actuator that is applied with an actuating current so as to control a hydraulically actuated fluid for driving the needle valve upwards and downwards, whereby the fuel to be injected out of the injector is regulated in injection timing and volume of injected fuel per cycle by a controller unit in response to the operating conditions of the engine.
There have been conventionally known two types of the injector used in the fuel-injection system, one of which is comprised of a solenoid-operated valve to control an ingress of the hydraulically actuated fluid, or hydraulic oil, into the injector body, and a boosting piston to pressurize the fuel in an intensified chamber, whereby the pressurized fuel makes the needle valve move so as to inject the pressurized fuel through the injection holes that have been free from the needle valve. Another type of the injector operates so as to regulate an ingress and egress of the highly pressurized fuel, which is accumulated in a common fuel supply rail, to a controlled pressure chamber in the injector body, whereby the pressurized fuel makes the needle valve move so as to inject the pressurized fuel through the injection holes that have been free from the needle valve.
FIG. 7 shows a prior fuel-injection system in which is incorporated the former type of the injector. The multicylinder engines, for example, four-cylinder or six-cylinder engine, have been dominated in most modern engines to attain the high horsepower. The injectors are each assigned to each cylinder to inject the fuel into the combustion chamber. In the fuel-injection system in FIG. 7, the fuel may be fed from a fuel tank 52 to a common fuel supply rail 51 through a fuel filter 54 by the driving of a fuel pump 53. The common fuel supply rail 51 is communicated with each of the injectors 1. It will be thus understood that the injectors 1 are constantly supplied with the fuel of the required pressure at their fuel inlets 11 and fuel outlets 12 through the common fuel supply rail 51. The unconsumed fuel remaining in each injector 1 may return to the fuel tank 52 through a recovery line 55.
The injectors 1 are supplied with the hydraulically actuating fluid, or high-pressurized oil, from a high-pressure fluid manifold 56 through a solenoid-operated valve 10. The high-pressure fluid manifold 56 is fed with the fluid in a fluid reservoir 57 through a fluid supply line 61 by the driving of a fluid pump 58. There are provided a fluid cooler 59 and a fluid filter 60 midway in the fluid supply line 61. Moreover the fluid supply line 61 is branched into a lubricant line 67 communicating with an oil gallery 62 and a hydraulic fluid line 66 communicated with pressure chambers 8, shown in FIG. 8, in the injectors 1. A hydraulic pump 63 is provided in the hydraulic fluid line 66 while a flow control valve 64 regulates the fluid supply to the high-pressure fluid manifold 56 from the hydraulic pump 63. A controller unit 50 is to control both of the flow control valve 64 and solenoids 10 of the injectors 1. The controller unit 50 is applied with data indicative of the operating conditions of an engine, that is, rotational frequencies detected by a rotational frequency sensor 68, throttle valve openings detected by a accelerometer 69 and crankshaft angles detected by a crank angle sensor 70. The controller unit 50 is also input with a hydraulic pressure in the high-pressure manifold 56, which is detected by a pressure sensor 71 in the high-pressure fluid manifold 56. The crank angles detected by the crank angle sensor 70 are available to control the beginning and duration of the electric conduction of the actuating current per cycle, in cooperation with signals from sensors indicative that a piston has reached the top dead center or the pre-determined position just before the top dead center of the compression phase at any standard cylinder or each cylinder.
FIG. 8 is an axial cross-sectioned view showing an exemplary injector 1 incorporated in the fuel-injection system in FIG. 7. The injector 1 is comprised of a nozzle body 2 formed at a distal end thereof with fuel-injection holes 13, a solenoid body 3 having mounted thereon a solenoid 15 serving as the electromagnetic actuator, an injector body 4 and a fuel supply body 5. The injector 1 further includes an intensified chamber supplied with fuel from the common fuel supply rail 51, a pressure chamber 8 supplied with a hydraulically actuating fluid, a boosting piston 9 actuated by the hydraulically actuated fluid from the pressure chamber 8 to apply the pressure to the fuel in the intensified chamber 7, a return spring 17 for forcing the boosting piston 9 to return to its neutral position, and a casing 6 having a fuel inlet 11 and a fuel outlet 12, which are communicated with the common fuel supply rail 51 to thereby provide a fuel chamber in the casing 6. In the injector 1 described just above, a needle valve 23 may move upwards and downwards by the action of the fuel pressure from the intensified chamber 7 to thereby open and close the injection holes 13. A solenoid-operated valve 10 has a valve body 16 that is actuated by the solenoid 15 to regulate the hydraulically actuated fluid supplied to the pressure chamber 8. The boosting piston 9 is composed of a radially-enlarged portion 25 and a radially-reduced portion 24, the former portion 25 being arranged for reciprocating movement in a first concave 26 in the injector body 4 and provided with a bottom face to define partially the pressure chamber 8, and the latter portion 24 being arranged for reciprocating movement in a second concave 27 and provided with a bottom face to define partially the intensified chamber 7.
FIG. 9 illustrates fuel-injection characteristics in the injectors, which are expressed as the coordinate relation of an actuating pulse width Pw versus an volume Q of fuel injected per cycle with taking a parameter of a hydraulic pressure in the high-pressure fluid manifold 56, or a rail pressure Pr. These characteristics may be obtained by the measurement of the volume Q of injected fuel per cycle with respect to the actuating pulse width Pw that is at least longer or equal to a pre-determined width. According to the characteristics, it will be seen that, as the actuating pulse width Pw increases, the duration when the injection holes are open becomes longer and then the volume Q of injected fuel per cycle increases. It will be further understood that the higher the rail pressure Pr is, the higher is the speed of opening the injection holes and the greater is the fuel-injection ratio so that the volume of injected fuel increases.
Disclosed in Japanese Patent Laid-Open No. 49591/1996 is an exemplary fuel-injection system, likewise with the system described above with reference to FIG. 7, and an injector adapted to be used in the system. The injector in the above citation is composed of a control valve, an intensifier and a nozzle. Moreover, Published Japanese translations on PCT international publication No. 511527/1994 discloses a similar fuel-injection system and an injector therefor. In these prior fuel-injection systems, controlling the electric conduction timing and duration to the electromagnetic actuator makes the fuel-injection start at the desired beginning of the fuel-injection and continue for the desired duration with the desired fuel-injection pressure, whereby the desired volume of fuel per cycle may be injected into the engine.
The prior injectors for the engines, as described above, are hard to be steady, but usually varied or scattered in the fuel-injection characteristic owing to the mechanical errors inevitably originating in working, assembly or the like of the components. For example, even if the solenoid-operated valve in the injector is kept at constant in the standard conductive duration thereto, the injectors each are uneven in their volumes of fuel injected per cycle. The Japanese Utility Model Publication No. 39037/1994 discloses, for example, a fuel supply system that has for its object to achieve the moderate fuel-injection control by compensating the uneven flow-rate characteristics in the fuel-injection valves, thereby preventing the deterioration in output and exhaust performances of the engine. In the prior fuel supply system in this citation, the fuel-injection valves are previously divided into plural subgroups in accordance with the levels in the flow-rate characteristic. The engine is provided with a fuel-injection valve matching with any one selected subgroup and further provided with resistors each having a resistance value corresponding to each subgroup of the flow-rate characteristic. There is provided compensating means that may discriminate the flow-rate characteristic, depending on the resistance values of the resistors, to thereby compensate the pulse width of the injection pulse signal in response to the correction value corresponding to the associated flow-rate characteristic. The compensating means are further designed such that the fuel-injection valve may match with the subgroup of the medium flow-rate characteristic when the resistance value is in infinity.
To cope with the dispersion or scattering in fuel-injection characteristic of the injectors, although the improvement in working accuracy of the components in the injectors is any one of means for reducing the dispersion or scattering in the fuel-injection characteristic, it is very hard to completely eliminate such dispersion while improving the accuracy in working and assembly results in a steep rise in the production cost of the injector. It will be conceived to previously observe the data of the relation between the duration conductive to the solenoid-operated valve and the volume of the injected fuel at numerous plots for each of the individual injectors and store the resultant data into the controller unit. Nevertheless, this involves a major problem such that enormous efforts are required to take the data and the controller unit must carry out the vast steps of calculation, resulting in raising the production cost for not only the injector but also the fuel-injection system having incorporated the injector therein.
Instead of previous observation of the fuel-injection characteristics at all plotting areas for the individual injectors, it will be conceivable that the required fuel-injection control may be realized inexpensively by correcting the fuel-injection characteristic in only the standard injector to regulate the fuel-injection of the individual injectors. That is, even if there is the dispersion or scattering for each injector in the fuel-injection characteristic regarding the relation between the standard conductive duration of the actuating current to the electromagnetic actuator and the volume of fuel injected out of the injection holes, the standard fuel-injection (reference fuel-injection) characteristic is assigned beforehand to the standard (reference) injector having, for example, the central value of dispersion or scattering in fuel-injection characteristic. The controller unit may be stored with only the standard fuel-injection characteristic in place of the individual fuel-injection characteristics in each injector. With attention to a definite correlation between the standard fuel-injection characteristic in the standard injector regarding the relation of the standard (reference) conductive duration of the actuating current versus the volume of injected fuel, and the fuel-injection characteristics in the individual injectors regarding the relation of the standard conductive duration of the actuating current versus the volume of injected fuel, for example, a proportional correlation of the standard conductive duration versus the volume of injected fuel, the definite correlation may be found out from the information relating to a specific point in the fuel-injection characteristic of the individual injectors. Hence, the standard conductive duration of the actuating current in the individual injectors may be determined by the correction of the standard fuel-injection characteristic, depending on the definite correlation.
In general, when the operating load in the engine detected as the depression of an accelerator pedal undergoes a change, the pressure in the hydraulically actuated fluid forced out from the pump varies while the standard conductive duration of the actuating current to the solenoid-operated valve is made longer or shorter so that the volume of the injected fuel may increase or decrease. It is true that the correction of the standard conductive duration defined in a pressure range of the hydraulically actuated fluid is usually different from that in another pressure range of the fluid. With the hydraulically actuated fluid undergoing a pressure change at a pressure range between pressure ranges different from each other, the standard conductive duration varies stepwise and therefore the actual volume of injected fuel undergoes a steep change while the torque from the engine also varies suddenly to thereby cause what is known as torque-shock. It is thus preferred that the standard conductive duration of the actuating current is kept from its steep change even under the pressure variation in the hydraulically actuated fluid whereby the engine may be protected from the sudden changes in its output power.